Operation and stabilization of Mod-MUX WDM transmitters based on silicon microrings

ABSTRACT

A transmitter comprising a plurality of modulator and multiplexer (Mod-MUX) units, each Mod-MUX unit operating at an optical wavelength different from the other Mod-MUX units. The transmitter can additional include in each Mod-MUX unit two optical taps and three photodetectors that are configured to allow the respective Mod-MUX unit to be tuned to achieve thermal stabilization and achieve effective modulation and WDM operation across a range of temperatures. The Mod-MUX transmitter avoids the use of a frequency comb. The Mod-MUX transmitter avoids cross-modulation between different modulators for different laser signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 14/514,771, filed Oct. 15, 2014, now allowed, whichclaims priority to U.S. Provisional Application No. 61/891,025, filedOct. 15, 2013, each of which is hereby incorporated by reference hereinin its entirety.

FIELD OF THE INVENTION

The invention relates to optical transmitters in general andparticularly to an optical transmitter that employs wavelength divisionmultiplexing.

BACKGROUND OF THE INVENTION

Wavelength division multiplexing (WDM) system has attracted more andmore interest in the past several years for building ultra-highaggregated data rate optical network and optical interconnects, giventhat interconnection has been considered as the bottleneck for thenext-generation computing systems. Microring resonators are one of themost popular devices to form the important building blocks of on-chipnetwork and optical interconnects, owing to their small footprint, smallcapacitance and low power consumption. Much progress has been made inthe past decade in designing and demonstrating microring-basedmodulators, filters, switches, lasers, and other structures.

The commonly used ring-based WDM transmitter architecture is shown inFIG. 1, in which a series of ring modulators share one bus waveguide.This architecture is referred to as “common-bus” architecture. Thisconfiguration does not require each ring modulator to be associated witha specific wavelength in the WDM system. Instead it offers theflexibility of assigning rings to the closest wavelength so as tominimize the overall tuning power. However, a comb laser orpre-multiplexed laser sources are required at the common input andcross-modulation may be introduced since the light in the bus waveguidepasses through multiple ring resonator modulators.

Automated thermal stabilization is particularly challenging in thecommon-bus design, due to the fact that multiple wavelengths are alwayspresent at the bus waveguide and interact with each ring modulator butthe monitoring photo detector is naturally insensitive to wavelength.

There is a need for an improved apparatus for multiplexing a pluralityof wavelengths onto a common optical fiber.

SUMMARY OF THE INVENTION

According to one aspect, the invention features an optical modulatorsystem. The system comprises a plurality of input stages, each inputstage of the plurality of input stages configured to operate at anoptical wavelength distinct from the optical wavelengths of operation ofthe others of the plurality of input stages, each input stage comprisingan optical input port configured to receive a light having a distinctoptical wavelength from a laser, a first modulator configured tomodulate the light having a distinct optical wavelength with informationcarried by a modulation signal to produce a modulated light signal onthe distinct optical wavelength as a carrier wavelength, and a filtermultiplexer configured to add the modulated light signal on the distinctoptical wavelength as a carrier wavelength onto an optical busconfigured to carry at least two light signals having different carrierwavelengths.

In one embodiment, the optical modulator system further comprises in atleast one of the plurality of input stages a first signal splittersituated between the optical input port and the first modulator and afirst optical detector configured to receive optical illumination fromthe first signal splitter; a second signal splitter situated between thefirst modulator and the filter multiplexer; and a second opticaldetector configured to receive optical illumination from the secondsignal splitter, and a third optical detector configured to receiveoptical illumination at a location beyond the filter multiplexer.

In another embodiment, the optical modulator system further comprises anoptical bus having an output port in optical communication with each ofthe filter multiplexers, the optical bus configured to provide amultiplexed optical signal comprising the respective modulated lightsignals on the distinct optical wavelengths as carrier wavelengths as anoutput signal.

In yet another embodiment, the optical modulator is configured tooperate according to a protocol selected from the group of protocolsconsisting of OOK, ASK, PSK, FSK, and PolSK.

In still another embodiment, the optical modulator is configured tooperate as a transmitter and as a receiver.

In a further embodiment, the optical modulator is permitted to drift inwavelength, and a communicating optical receiver is tuned to receive thewavelength that drifts.

According to another aspect, the invention relates to a method ofcontrolling a thermal regime in an optical modulator system. The methodcomprises providing an optical modulator system comprising a pluralityof input stages, each input stage of the plurality of input stagesconfigured to operate at an optical wavelength distinct from the opticalwavelengths of operation of the others of the plurality of input stages,each input stage comprising: an optical input port configured to receivea light having a distinct optical wavelength from a laser, a firstmodulator configured to modulate the light having a distinct opticalwavelength with information carried by a modulation signal to produce amodulated light signal on the distinct optical wavelength as a carrierwavelength, and a filter multiplexer configured to add the modulatedlight signal on the distinct optical wavelength as a carrier wavelengthonto an optical bus configured to carry at least two light signalshaving different carrier wavelengths; in at least one of the pluralityof input stages a first signal splitter situated between the opticalinput port and the first modulator and a first optical detectorconfigured to receive optical illumination from the first signalsplitter; a second signal splitter situated between the first modulatorand the filter multiplexer; and a second optical detector configured toreceive optical illumination from the second signal splitter, and athird optical detector configured to receive optical illumination at alocation beyond the filter multiplexer; in a respective one of theplurality of input stages, observing the photocurrents on the first andsecond optical detectors; tuning a thermal tuner on the first modulatorto achieve a desired ratio of photocurrents in the first opticaldetector and the second optical detector; and thereafter, tuning athermal tuner on the filter multiplexer to minimize a photocurrent inthe third optical detector, thereby maintaining operation of the opticalmodulator system over a range of temperatures.

According to another aspect, the invention relates to a method ofoperating an optical modulator system. the method comprises providing anoptical modulator system comprising a plurality of input stages, eachinput stage of the plurality of input stages configured to operate at anoptical wavelength distinct from the optical wavelengths of operation ofthe others of the plurality of input stages, each input stagecomprising: an optical input port configured to receive a light having adistinct optical wavelength from a laser, a first modulator configuredto modulate the light having a distinct optical wavelength withinformation to produce a modulated light signal on the distinct opticalwavelength as a carrier wavelength, and a filter multiplexer configuredto add the modulated light signal on the distinct optical wavelength asa carrier wavelength onto an optical bus configured to carry at leasttwo light signals having different carrier wavelengths; providing toeach of the plurality of input stages an optical input signal as arespective carrier wave, the respective carrier waves having arespective distinct optical wavelength; providing to each of theplurality of input stages a modulation signal that carries informationto be modulated onto the carrier wave; and recovering a modulated signalhaving at least one of the respective carrier waves having therespective distinct optical wavelength at an output of the optical bus.

In one embodiment, the method further comprises providing at least oneoptical detector configured to receive an optical signal having aspecific carrier wavelength in optical communication with a respectiveone of the filter multiplexers; receiving a modulated signal having thespecific carrier wavelength at the filter multiplexer from the opticalbus; detecting the modulated signal having the specific carrierwavelength with the at least one optical detector; and receiving fromthe at least one optical detector a signal comprising informationencoded on the modulated signal, free of the specific carrierwavelength, which signal is configured to be displayed to a user,recorded in a non-volatile memory and/or transmitted to another devicefor further manipulation.

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more apparent from the following descriptionand from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood withreference to the drawings described below, and the claims. The drawingsare not necessarily to scale, emphasis instead generally being placedupon illustrating the principles of the invention. In the drawings, likenumerals are used to indicate like parts throughout the various views.

FIG. 1 is a diagram that illustrates a traditional prior art common busarchitecture.

FIG. 2 is a schematic diagram that illustrates the apparatus andprinciple of operation of the Mod-MUX WDM transmitter architectureaccording to principles of the invention in which CW sources are fedinto a ring modulator, and the modulated light is multiplexed onto thebus waveguide by a ring filter.

FIG. 3 is a schematic diagram that illustrates the apparatus andprinciple of operation of a Mod-MUX WDM transmitter according toprinciples of the invention that provides a convenient method ofautomated thermal stabilization.

FIG. 4 is a schematic diagram that illustrates a Mod-MUX WDM transmitterarchitecture according to principles of the invention having multiplerings as the multiplexer unit.

FIG. 5 is an image of a chip on which a Mod-MUX transmitter according toprinciples of the invention has been fabricated.

DETAILED DESCRIPTION

We describe the Mod-MUX architecture for implementing a WDM transmitterusing ring modulators. As compared to the conventional ‘common-bus’architecture, the Mod-MUX architecture shows many advantages. We alsodescribe a procedure that can be used to thermally stabilize the Mod-MUXtransmitter automatically.

The Mod-MUX architecture o overcomes the weakness of the common-busdesign. As shown in FIG. 2, in the Mod-MUX architecture the laser foreach channel is first fed into a ring modulator (Mod) and then themodulated light is multiplexed onto the bus waveguide by a ringadd-filter multiplexer (MUX). In FIG. 2, the first Mod-MUX operates atwavelength λ₁, the second Mod-MUX operates at wavelength λ₂, and then^(th) Mod-MUX operates at wavelength λ_(n), where n is an integergreater than 1 that defines the number of discrete wavelengths that theMod-MUX can accommodate.

In FIG. 2, each Mod-MUX has a respective optical input (210, 220, 2 n 0)that receives a carrier signal having wavelength different from theother carrier signal wavelengths, a respective modulator (211, 221, 2 n1) that modulates the respective carrier signal with a signal inresponse to a data signal (e.g., information carried by a modulationsignal) applied to the modulator to produce a modulated carrier signal,and a respective multiplexer (212, 222, 2 n 2) that multiplexers themodulated carrier signal onto a bus 230, so that the combined modulatedsignals are provided at an output 240. The output signal can berecorded, transmitted to another device, or displayed to user, possiblyin digital form or in the form of an active/inactive indicatorcorresponding to a given wavelength. For convenience, a modulator and amultiplexer (a Mod-MUX) that operates on a single carrier signal havinga specific wavelength as described may be referred to as a Mod-MUX unit.

This architecture offers several advantages. It removes the requirementof providing a comb source. It avoids cross-modulation, due to the facteach laser wavelength only passes through one ring modulator. TheMod-MUX offers compatibility to simpler thermal stabilization schemescompared to the common-bus architecture since each Mod-MUX branchoperates with only one laser wavelength. A specific design can beprovided for ring modulators and ring filters respectively to optimizethe performance of each element, such as the best tunability with themaximum allowable quality factor (in the ring modulator) and sufficientbandwidth with low loss and low cross-talk (in the ring filter).

For proper operation, one should preferably monitor the optical power atthe bus output when sending a tunable CW light into each input asillustrated in FIG. 2. One first tunes the filters to achieve the targetchannel spacing, and then one tunes the modulator resonances toapproximately align with the respective filter. In principle, it ispreferable that the laser wavelength should be at the peak of theoptical filter to minimize loss and optical filtering of the datastream, and the modulator resonance preferably should be slightly offthe laser wavelength just as it is in a single ring modulator togenerate a desired extinction ratio.

In various embodiments, each modulator element may be a single ring or amulti ring. In other embodiments, the modulator element may be adifferent type of modulator such as a Mach Zehnder Interferometer (MZI),an electro-absorptive (EA) optical modulator, or a modulator of anothertype.

In some embodiments, the modulator elements may be specifically designedto thermally drift together (for instance by placing them physicallyclose to one another in order to match them) so as to simplify theconstruction of a control system.

In some embodiments, the Mod-MUX transmitters may include additionaltaps, detectors, and similar elements useful for the creation of controlsystems.

In some embodiments, there may also be elements added to the rings, suchas thermal tuners (heating and/or cooling elements), PIN junctiontuners, and the like, in order to provide a “control knob” by which tocontrol such operating parameters as operating temperature, biassignals, and the like.

In some embodiments, the resonators may be rings, disks, or otherstructures such as various linear cavities.

Automated Thermal Stabilization of Mod-MUX Transmitter

Turning to FIG. 3, we now present an apparatus and a procedure toachieve automated thermal tuning of a Mod-MUX transmitter. The firstMod-MUX that operates at wavelength λ₁ includes input 300 where acarrier signal at wavelength λ₁ is introduced, modulator 301, MUX 302,tap 303, photodetector 304, tap 305, photodetector 306, andphotodetector 307. The n^(th) Mod-MUX that operates at wavelength λ_(n)includes input 3 n 0 where a carrier signal at wavelength λ_(n) isintroduced, modulator 3 n 1, MUX 3 n 2, tap 3 n 3, photodetector 3 n 4,tap 3 n 5, photodetector 3 n 6, and photodetector 3 n 7. Bus 320receives the respective modulated signals on carrier signals atwavelengths λ₁, λ₂, . . . , λ_(n) and provides the multiplexed result atan output port 330. In FIG. 3 Mod-MUX units that operate at only the twowavelengths λ₁ and λ_(n) are illustrated. However, it should beunderstood that any convenient number of Mod-MUX units operating atdifferent distinct wavelengths can be provided. In some embodiments, thetaps are 95/5 taps. One can use other tap ratios, so long as the twotaps are matched, and enough illumination intensity is provided so thatthe intensity can be measured while the intensity passed through thetaps to the modulator is adequate. As seen in FIG. 3, the two taps (303,305 and 3 n 3, 3 n 5) are inserted before and after the respective ringmodulator 301, 3 n 1. The tapped out light intensity is fed intomonitoring PDs (304, 306 and 3 n 4, 3 n 6) respectively. The thirdmonitoring PD (307, 3 n 7) is connected to the through output of therespective ring filter 302, 3 n 2. Applying the following procedure oneach branch will tune the ring modulators and filters so the transmittercan work properly.

For a selected Mod-MUX unit (for example, the leftmost unit in FIG. 3):

-   -   1. Tune the thermal turner on the ring modulator 301 and monitor        the photo current I_(a) and I_(b) on the monitoring PD 304 (a)        and PD 306 (b), respectively.    -   2. Stop tuning when I_(b)/I_(a) achieves the desired bias loss.    -   3. Tune the thermal tuner on the ring filter, so that the photo        current L on monitoring PD 307 (c) is minimized.

This technology enables the following:

-   -   1. The use of a two-level series of cascaded ring modulators to        achieve wavelength-division modulation (WMD) and multiplexing        (muxing) (e.g., the combination of optical signals at different        wavelengths on a single optical fiber) as shown in FIG. 2, via a        layer of independent ring modulators, followed by a set of ring        modulators used to mux the individual signals on to a common        optical bus.    -   2. The avoidance of a requirement for a comb source for WDM        transmission based on a two-layer set of ring modulators, with        one layer used for modulation, and the second layer for        multiplexing.    -   3. The utilization of the architecture described herein to avoid        cross-modulation between different ring modulators for different        laser signals.    -   4. The utilization of the algorithm described in the section        “Automated Thermal Stabilization of Mod-MUX Transmitter” to        thermally stabilize a two-layer set of ring modulators and        achieve effective modulation and WDM operation across a range of        temperatures.

FIG. 4 is a schematic diagram that illustrates a Mod-MUX WDM transmitterarchitecture according to principles of the invention having multiplerings as the multiplexer unit.

There are two advantages provided by the use of multiple rings in themultiplexer. First, the free spectrum range (FSR) of the two coupledrings will be much larger than a single ring due to the vernier effect,which expands the operation wavelength range of the transmitter. Second,the wavelength selection ability of two coupled rings is better than asingle ring because the two ring filter is a higher order filter. Inother words, the interference between the neighboring channels can bereduced.

FIG. 5 is an image of a chip on which a Mod-MUX transmitter according toprinciples of the invention has been fabricated.

The transmitter was fabricated in a CMOS compatible photonics foundry.The process starts with an 8″ Silicon-on-Insulator (SOI) wafer fromSOITEC with 220 nm top silicon and 2 μm bottom oxide thickness. Ahigh-resistivity handle silicon (750 Ω·cm) was used to ensure the RFperformance. Grating couplers and silicon waveguides were formed bythree dry etches. Six implantation steps were applied to silicon to formthe pn junction and contact region. Two layers of aluminum weredeposited for electrical interconnection. In all cases, 248 nmphotolithography was utilized.

The fabrication process is further described in Liu, Yang, et al. “30GHz silicon platform for photonics system.” Optical InterconnectsConference, May 5, 2013, IEEE, and in Liu, Yang, et al. “SiliconMod-MUX-Ring transmitter with 4 channels at 40 Gb/s.” Optics Express 22(2014): 16431-16438, Jun. 25, 2014, each of documents is herebyincorporated by reference herein in its entirety.

Applications

The present application can be used with well-known methods oftransmitting information over optical communication networks. Forexample, such systems and methods are discussed in I. Djordjevic et al.,Coding for Optical Channels, Chapter 2, Fundamentals of OpticalCommunication, pages 25-73, Springer, 2010, ISBN 978-1-4419-5569-2,which is said to describe optical components, different modulationformats with direct detection, and different modulation schemes withcoherent detection, which document is hereby incorporated by referenceherein in its entirety.

In order to exploit the enormous bandwidth potential of optical fibersystems, different multiplexing techniques (OTDMA, WDMA, CDMA, SCMA),modulation formats (OOK, ASK, PSK, FSK, PolSK, CPFSK, DPSK, etc.),demodulation schemes (direct detection or coherent), and technologiescan be employed.

Two types of external modulators commonly used in practice: Mach-Zehndermodulator (MZM) and electroabsorption modulator (EAM). Possiblemodulation formats that can be used with a MZM include: on-off keying(OOK) with zero/nonzero chirp, binary phase-shift keying (BPSK),differential phase-shift keying (DPSK), quadrature phase-shift keying(QPSK), differential QPSK (DQPSK), and return-to-zero (RZ) with dutycycle 33%, 50%, or 67%.

Basic optical modulation formats can be categorized as follows (1)On-Off Keying (OOK), where the 1 is represented by the presence of thepulse while the 0 by the absence of a pulse; (2) Amplitude-shift keying(ASK), where the information is embedded in the amplitude of thesinusoidal pulse; (3) Phase-shift keying (PSK), where the information isembedded in the phase; (4) Frequency-shift keying (FSK), where theinformation is embedded in the frequency; and (5) Polarization-shiftkeying (PolSK), where the information is embedded in the polarization.Various modulation formats can be used with direct detection, namely (1)non-returnto-zero (NRZ), (2) return-to-zero (RZ), (3) alternate markinversion (AMI), (4) duobinary modulation, (5) carrier-suppressed RZ,(6) NRZ-differential phase-shift keying (NRZ-DPSK), and (7)RZ-differential phase-shift keying (RZ-DPSK).

Design and Fabrication

Methods of designing and fabricating devices having elements similar tothose described herein are described in one or more of U.S. Pat. Nos.7,200,308, 7,339,724, 7,424,192, 7,480,434, 7,643,714, 7,760,970,7,894,696, 8,031,985, 8,067,724, 8,098,965, 8,203,115, 8,237,102,8,258,476, 8,270,778, 8,280,211, 8,311,374, 8,340,486, 8,380,016,8,390,922, 8,798,406, and 8,818,141, each of which documents is herebyincorporated by reference herein in its entirety.

Operation

In some embodiments, the Mod-MUX transmitters of the invention are ableto be used with various modulation formats, including OOK, ASK, PSK,FSK, and PolSK.

In some embodiments, the Mod-MUX transmitters of the invention can beallowed to drift in wavelength, rather than being thermally stabilized.In such embodiments the communicating receiver tunes itself in order tolock to the incoming wavelengths.

In some embodiments, the Mod-MUX transmitters of the invention can beused as either or both of a transmitter and a receiver. The transmitterstructure has been described. For use as a receiver, at least one of theWDM rings leads to a detector, which can receive a demultiplexed signalfrom the optical bus. For bidirectional operation, e.g., as atransmitter and as a receiver (a transceiver), some of the MUX ringslead to modulators and some lead to detectors. For example, in FIG. 3,in one considers the first Mod-MUX that operates at wavelength λ₁, onecould turn off the source of the carrier signal to input 300. One couldrecover a signal having a carrier at wavelength λ₁ from the bus 320using MUX 302. One could detect the signal using any of tap 303 andphotodetector 304, tap 305 and photodetector 306, and photodetector 307.One could thereby recover the information encoded on the carrier wave ata wavelength λ₁.

Definitions

Recording the results from an operation or data acquisition, such as forexample, recording results at a particular frequency or wavelength, isunderstood to mean and is defined herein as writing output data in anon-transitory manner to a storage element, to a machine-readablestorage medium, or to a storage device. Non-transitory machine-readablestorage media that can be used in the invention include electronic,magnetic and/or optical storage media, such as magnetic floppy disks andhard disks; a DVD drive, a CD drive that in some embodiments can employDVD disks, any of CD-ROM disks (i.e., read-only optical storage disks),CD-R disks (i.e., write-once, read-many optical storage disks), andCD-RW disks (i.e., rewriteable optical storage disks); and electronicstorage media, such as RAM, ROM, EPROM, Compact Flash cards, PCMCIAcards, or alternatively SD or SDIO memory; and the electronic components(e.g., floppy disk drive, DVD drive, CD/CD-R/CD-RW drive, or CompactFlash/PCMCIA/SD adapter) that accommodate and read from and/or write tothe storage media. Unless otherwise explicitly recited, any referenceherein to “record” or “recording” is understood to refer to anon-transitory record or a non-transitory recording.

As is known to those of skill in the machine-readable storage mediaarts, new media and formats for data storage are continually beingdevised, and any convenient, commercially available storage medium andcorresponding read/write device that may become available in the futureis likely to be appropriate for use, especially if it provides any of agreater storage capacity, a higher access speed, a smaller size, and alower cost per bit of stored information. Well known oldermachine-readable media are also available for use under certainconditions, such as punched paper tape or cards, magnetic recording ontape or wire, optical or magnetic reading of printed characters (e.g.,OCR and magnetically encoded symbols) and machine-readable symbols suchas one and two dimensional bar codes. Recording image data for later use(e.g., writing an image to memory or to digital memory) can be performedto enable the use of the recorded information as output, as data fordisplay to a user, or as data to be made available for later use. Suchdigital memory elements or chips can be standalone memory devices, orcan be incorporated within a device of interest. “Writing output data”or “writing an image to memory” is defined herein as including writingtransformed data to registers within a microcomputer.

Theoretical Discussion

Although the theoretical description given herein is thought to becorrect, the operation of the devices described and claimed herein doesnot depend upon the accuracy or validity of the theoretical description.That is, later theoretical developments that may explain the observedresults on a basis different from the theory presented herein will notdetract from the inventions described herein.

Any patent, patent application, patent application publication, journalarticle, book, published paper, or other publicly available materialidentified in the specification is hereby incorporated by referenceherein in its entirety. Any material, or portion thereof, that is saidto be incorporated by reference herein, but which conflicts withexisting definitions, statements, or other disclosure materialexplicitly set forth herein is only incorporated to the extent that noconflict arises between that incorporated material and the presentdisclosure material. In the event of a conflict, the conflict is to beresolved in favor of the present disclosure as the preferred disclosure.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawing, itwill be understood by one skilled in the art that various changes indetail may be affected therein without departing from the spirit andscope of the invention as defined by the claims.

What is claimed is:
 1. A WDM transmitter system, comprising: an opticalbus including a port for outputting a combined modulated signal; a firstlaser for generating a first optical signal at a first opticalwavelength; a first tunable modulator configured to modulate said firstoptical signal with information carried by a first modulation signal toproduce a first modulated light signal, the first tunable modulatorconfigured to be tunable based on intensities of at least one of thefirst optical signal before the first tunable modulator or the firstmodulated light signal after the first tunable modulator; a first filtermultiplexer configured to add said first modulated light signal onto theoptical bus; a second laser for generating a second optical signal at asecond optical wavelength; a second tunable modulator configured tomodulate said second optical signal with information carried by a secondmodulation signal to produce a second modulated light signal, the secondtunable modulator configured to be tunable based on intensities of atleast one of the second optical signal before the second tunablemodulator or the second modulated light signal after the second tunablemodulator; a second filter multiplexer configured to add the said secondmodulated light signal onto the optical bus with the first modulatedlight signal to produce the combined modulated signal; a first signalsplitter situated between said first laser and said first tunablemodulator for tapping off a portion of the first optical signal; a firstoptical detector configured to receive the portion of the first opticalsignal for generating a first photo current; a second signal splittersituated between said first tunable modulator and said first filtermultiplexer for tapping of a portion of the first modulated lightsignal; a second optical detector configured to receive the portion ofthe first modulated light signal for generating a second photo current;wherein the first tunable modulator is configured to be tuned until afirst desired loss is achieved based on at least one of the first andsecond photo currents; a third optical detector configured to receiveoptical illumination of the first modulated light signal at a locationbeyond said first filter multiplexer; and a tuner configured for tuningthe first filter multiplexer to minimize loss of the first modulatedlight signal based on the optical illumination received at the thirdoptical detector.
 2. The WDM transmitter system of claim 1, wherein eachof the first and second filter multiplexers comprises a micro-ringfilter multiplexer; wherein each of the first and second tunablemodulators comprises a micro-ring modulator; and wherein the first andsecond filter multiplexers, and the first and second tunable modulatorsare disposed on a single chip.
 3. The WDM transmitter system of claim 1,wherein the first tunable modulator is configured to be tunable toprovide a modulator resonance slightly off the first optical wavelengthto generate a first desired extinction ratio.
 4. The WDM transmittersystem of claim 3, wherein the second tunable modulator is configured tobe tunable to provide a modulator resonance slightly off the secondoptical wavelength to generate a second desired extinction ratio.
 5. TheWDM transmitter system of claim 1, wherein the tuner is configured fortuning the first filter multiplexer until the first optical wavelengthis at a peak of the first filter multiplexer.
 6. The WDM transmittersystem of claim 1, wherein the third optical detector generates a thirdphoto current from the optical illumination at the location beyond saidfilter multiplexer; wherein the tuner is configured for tuning the firstfilter multiplexer until the third photo current is minimized.
 7. TheWDM transmitter system of claim 1, wherein the first filter multiplexercomprises a multiple ring filter to increase an FSR of the first filterelement and reduce interference between the first and second modulatedlight signals.
 8. The WDM transmitter system of claim 1, furthercomprising: a third laser for generating a third optical signal at athird optical wavelength, a third tunable modulator configured tomodulate said third optical signal with information carried by a thirdmodulation signal to produce a third modulated light signal, the thirdtunable modulator configured to be tunable to provide a modulatorresonance wavelength to generate a third desired extinction ratio; athird filter multiplexer configured to add the third modulated lightsignal onto the optical bus with the first and second modulated lightsignals to produce the combined modulated signal.
 9. The WDM transmittersystem of claim 1, further comprising: a first filter demultiplexerconfigured to receive an input optical signal from the optical bus; anda detector coupled to the first filter demultiplexer for recoveringinformation encoded on the input optical signal.
 10. A method ofoperating a WDM transmitter system, comprising: generating a firstoptical signal at a first optical wavelength using a first laser;modulating said first optical signal with information carried by a firstmodulation signal to produce a first modulated light signal using afirst tunable modulator; tuning the first tunable modulator based onintensities of at least one of the first optical signal before the firsttunable modulator or the first modulated light signal after the firsttunable modulator; adding said first modulated light signal onto theoptical bus using a first filter multiplexer; generating a secondoptical signal at a second optical wavelength using a second laser;modulating said second optical signal with information carried by asecond modulation signal to produce a second modulated light signalusing a second tunable modulator; tuning the second tunable modulatorbased on intensities of at least one of the second optical signal beforethe second tunable modulator or the second modulated light signal afterthe second tunable modulator; adding the second modulated light signalonto the optical bus with the first modulated light signal to producethe combined modulated signal using a second filter multiplexer; tappingoff a portion of the first optical signal using a first signal splittersituated between said first laser and said first tunable modulator;generating a first photo current in a first optical detector configuredto receive the portion of the first optical signal; tapping off aportion of the first modulated light signal using a second signalsplitter situated between said first tunable modulator and said firstfilter multiplexer; and generating a second photo current in a secondoptical detector configured to receive the portion of the firstmodulated light signal; tuning the first tunable modulator until a firstdesired loss is achieved based on at least one of the first and secondphoto currents; receiving optical illumination of the first modulatedlight signal using a third optical detector a location beyond said firstfilter multiplexer; and tuning the first filter multiplexer to minimizeloss of the first modulated light signal based on the opticalillumination received at the third optical detector.
 11. The method ofclaim 10, wherein the step of tuning the first tunable modulatorcomprises tuning the first tunable modulator to provide a firstmodulator resonance slightly off the first optical wavelength togenerate a first desired extinction ratio.
 12. The method of claim 11,wherein the step of tuning the second tunable modulator comprises tuningthe second tunable modulator to provide a second modulator resonanceslightly off the second optical wavelength to generate a second desiredextinction ratio.
 13. The method of claim 10, wherein the step of tuningthe first filter multiplexer includes tuning the first filtermultiplexer until the first optical wavelength is at a peak of the firstfilter multiplexer.
 14. The method of claim 10, further comprising:generating a third photo current in the third optical detector from theoptical illumination at the location beyond said filter multiplexer;wherein the step of tuning the first filter multiplexer comprises tuningthe first filter multiplexer until the third photo current is minimized.15. The method of claim 10, further comprising: receiving an inputoptical signal from the optical bus at a first filter demultiplexer; andrecovering information encoded on the input optical signal using adetector coupled to the first filter demultiplexer.